May 27, 2013 | 2
Giardia is a cute flagellate with two nuclei, eight flowing flagella and an impressive sucker plate that makes it look rather like a catfish. Their elegant swimming patterns are reminiscent of one as well. Giardia also lacks canonical mitochondria, instead harbouring highly reduced derivatives called mitosomes. Thus, it’s not particularly fond of oxygen — recall that mitochondria are involved in oxygen-based respiration. As far as lifestyles go, that usually means one of two things: either they love rotting black goo purged of much of the oxygen by the decay activity; or they inhabit some other critter. Thus, many anaerobes are symbionts: mutualists or parasites (or commensals). Note that by far, not all parasites and mutualists are anaerobes.
Oh, I should probably mention a minor detail: Giardia is notorious for giardiasis — also known as beaver fever — a nasty disease obtained from drinking outdoor — and otherwise contaminated — water, especially from slow-moving streams and water downstream of dead animals, faeces, etc. To normal, well-balanced people, this means Giardia is to be avoided at all costs, and that research must be done to either get rid of it or make the sickness marginally more bearable. To a protist nut, the clinical importance means we can have a better understanding of a phylogenetically unique and fascinating model organism. Excavates, the group containing Giardia, are diverse but not particularly well-understood. Naegleria and trypanosomes are fellow opportunists and parasites, respectively, who bring ire upon humans and attract scientific funding. Giardia is on the other branch of the most basic divide between groups of Excavates, however — closer to the enigmatic parabasalids and oxymonads of the termite gut. Oh, and they’re also literally double cells — two nuclei, and two sets of four flagella, in a mirrored arrangement. What more can a cell biologist want?
An impressive scanning EM collage of Giardia in division. (Nohýnková et al. 2006 Euk Cell; free access) Isn’t it cute?
As far as parasites go, Giardia has quite a simple overall lifestyle (PS: if you’re an instructor and make your students memorise parasitic life cycles — you are a sadist! ;p). Sex has not been observed, though some evidence hints that it might perhaps happen. Essentially, Giardia spends its life either lying in wait as a cyst, or leaving the cyst and swimming towards intestinal walls, attaching among the villi with a specialised suction plate. There it sits and feeds on the host’s tasty juices, like glucose. Apparently, intestines don’t really like having uninvited things attached to them, and all hell breaks loose. Not that the parasite is of a particularly friendly disposition either.
They’re cute under lasers too. Confocal image of Giardia: microtubules in red, the two nuclei in blue. (Sagolla et al 2006 J Cell Sci; free access)
Being highly derived — as a result of their phylogenetic position as well as parasitism — Giardia has unusual genome structure and cell biology. Giardia is both Golgi- and actin-challenged. The Golgi bodies are extremely reduced, and for a long time were thought to be absent altogether — though now it is known that their derived form is necessary for cyst formation (Faso et al. 2012 Cell Microbiol). Actin is present in a single copy, and a really weird one at that. Major standard actin-binding proteins are absent altogether (Paredez et al. 2011 PNAS; free access), including the motor protein myosin (you might know it from muscles). The cell skeleton component actin is present in all eukaryotes, and plays a major role in things like growth and amoeboid movement. Curiously, a number of typically non-actin-regulating proteins have been found to be recruited to actin in Giardia — an example of functional replacement, possibly one that then enabled the loss typical actin-associated proteins (last part is speculation on my part).
Here’s an excellent electron micrograph showing the structure of the middle of the cell, with four of the flagella exposed. Note the finely-spaced contours on the lower left, fine enough to appear to form a diffraction grating at low resolution. More on that in a bit!
Image shamelessly stolen from: Cande Lab website/Images (by Joel Mancuso). Image modified by slight unsharp mask and then halving the size.
Before I go on to ultrastructure, I must mention an important note — it is commonly stated, even in scientific literature, that Giardia is ancient. This is not the case: the statement is based on old data from the 90s, which were based on outdated phylogenetic techniques. This misconception dates back to the Archezoa Hypothesis, wherein a mitochondrion was gradually growing in sophistication along the anaerobic deep-branching eukaryotes (on the “bottom” of the tree), until it reached its canonical complexity at the base of the divergence of more ‘normal’ eukaryotes — like algae, ciliates and the “crown” group of Fungi, Animals and Plants.
In the early 2000s, this hypothesis fell apart as newer tree-building techniques were better at resolving highly derived (unusual) critters, and all of the mitochondrially-challenged anaerobes were placed in the midst of mitochondria-bearing groups. Furthermore, mitochondrial genes were found in each of those anaerobes, confirming that proper genome-bearing mitochondria came first. Thus, Giardia is in no way unusually ancient, any more so than the other eukaryotes, and is claimed as such out of pure inertia. Public service announcement over.
Suction is used for attachment, but it is still not entirely clear how that suction (negative pressure) is generated (House et al. 2011 PLoS Path; free access). Remember the first electron micrograph above, and the very fine lines at the bottom left? These are part of a section of the sucker plate(=ventral disk, more technically). Those bands are made of cytoskeletal elements, and are thought to be the stuff that generates suction — namely, microtubules. Giardia, despite (because of?) its modest actin repertoire, has an elaborate microtubular cytoskeleton. The ultimate focus of this post (I stick to ‘short’ introductions, y’see), an electron tomography study by Schwartz et al. (2012 PLoS ONE; free access), reveals the most in-depth information about this complex organelle’s structure seen thus far.
Electron tomography(ET) uses thicker sections than classical transmission electron microscopy, and renders 3D reconstructions within those sections by imaging it from different directions. This allows structures to to be inferred in 3D without losing as much data to sectioning — though a bit of resolution does get sacrificed. The 3D sections are then assembled to form a computer model of the whole organism, or its chunks of interest. ET has already recently revealed plenty of structures that were previously unseen or misinterpreted — it’s not always obvious whether and where one blob meets up with another in thin-section electron microscopy. And pretty models are generated — I will post some more later. As a deeply serious scientist, I like pretty pictures =)
The microtubules are nucleated(=rooted) at the centre of the disk, and extend outwards in a loose spiral towards the edge. Atop the microtubules (ie, facing away from the bottom surface), ‘microribbons’ protrude into the cytoplasm. The ‘tubes and ribbons are spaced closer together towards the edge. The figure above shows the microtubule arrangement and polarity (their have a rapidly growing+shrinking and a slowly shrinking end, roughly speaking), and how they inferred it (don’t worry about that). I find it curious that while the cell is obsessively symmetrical, the disk itself is arranged in a spiral. Additionally, since the microtubules are uncapped at the slowly-shrinking end (often some protein is squished onto there to stop the shrinking) — are the ‘tubes in the disk highly dynamic, constantly forming, growing in a spiral and losing their shrinking end, being replaced with younger microtubules? You’d probably need superresolution live cell confocal microscopy for that. Would love to see the movies!
The next figure shows an insanely zoomed-in 3D reconstruction of a section of the microtubule-microribbon complex. The circular part comes from the microtubule being sliced across. It maps some individual proteins and major structures of the complex, and the point here that you care about is that it’s complex, and you can figure out where individual proteins go. The lowercase ‘g’ means it’s Giardia‘s version of the protein, while MAPs and MIPs are creatively-named Microtubule-Associated Proteins and Microtubule Inner Proteins, respectively. How they assemble is an even more exciting process, but don’t ask me. I’m in awe of how much structure can be seen in electron tomography!
In addition to presenting a stunning reconstruction of a complex protist structure, this paper presents a novel imagining technique (not the tomography, but the technical way they used it), which I’ll keep in mind for further reference, just in case. Reconstructing complicated 3D structures is kind of a fun thing, and shows us once again how far from simple both cell and protistan biology are!
Focus article: Schwartz CL, Heumann JM, Dawson SC, Hoenger A (2012) A Detailed, Hierarchical Study of Giardia lamblia’s Ventral Disc Reveals Novel Microtubule-Associated Protein Complexes. PLoS ONE 7(9): e43783. doi:10.1371/journal.pone.0043783
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